Foldable coil array

ABSTRACT

In one example, an RF coil array includes a first RF coil configured to generate a magnetic field along a first axis, the first RF coil having a first surface, a second RF coil configured to generate a magnetic field along a second axis, orthogonal to the first axis, the second RF coil having a second surface, and a first foldable interconnect coupling the first RF coil to the second RF coil. The first foldable interconnect may be adjusted to couple the first RF coil to the second RF coil with a first amount of overlap and with the first surface and second surface facing a common direction, or couple the first RF coil to the second RF coil with a second amount of overlap, larger than the first amount of overlap, and with the first surface in face to face position with the second surface.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 15/169,183, entitled “FOLDABLE COIL ARRAY”, and filed on May 31,2016, now U.S. Pat. No. 10,132,883. The entire contents of theabove-listed application are hereby incorporated by reference in itsentirety for all purposes.

FIELD

Embodiments of the subject matter disclosed herein relate tonon-invasive diagnostic imaging, and more particularly, to magneticresonance imaging systems.

BACKGROUND

Magnetic resonance imaging (MRI) is a medical imaging modality that cancreate images of the inside of a human body without using x-rays orother ionizing radiation. MRI uses a powerful magnet to create a strong,uniform, static magnetic field. When a human body, or part of a humanbody, is placed in the main magnetic field, the nuclear spins that areassociated with the hydrogen nuclei in tissue water or fat becomepolarized. This means that the magnetic moments that are associated withthese spins become preferentially aligned along the direction of themain magnetic field, resulting in a small net tissue magnetization alongthat axis. An MRI system also comprises components called gradient coilsthat produce smaller amplitude, spatially varying magnetic fields when acurrent is applied to them. Typically, gradient coils are designed toproduce a magnetic field component that is aligned along the z-axis andthat varies linearly in amplitude with position along one of the x, y,or z-axes. The effect of a gradient coil is to create a small ramp onthe magnetic field strength and, in turn, on the resonant frequency ofthe nuclear spins along a single axis. Three gradient coils withorthogonal axes are used to “spatially encode” the MRI signal bycreating a signature resonance frequency at each location in the body.Radio frequency (RF) coils are used to create pulses of RF energy at ornear the resonance frequency of the hydrogen nuclei. The RF coils areused to add energy to the nuclear spins in a controlled fashion. As thenuclear spins then relax back to their rest energy state, they give upenergy in the form of an RF signal. This signal is detected by the MRIsystem and is transformed into an image using a computer and knowreconstruction algorithms.

Sizing of RF coil arrays may include a trade-off between adequatecoverage of a region of interest to be imaged and suitablesignal-to-noise ratios. For example, RF coil arrays need to be largeenough to receive MR signals from the anatomy region of interest. On theother hand, the RF coil arrays cannot be made too large, otherwise thesignal to noise ratio (SNR) of the arrays will be degraded due to thepoor fitting factor of large coil arrays and large size of coilelements. This trade-off is complicated by the large variability ofpatient sizes. A given RF coil array may be too small to providesufficient coverage to large size patients, yet the same coil array maybe too big to fit the small size patients and result in low SNR or poorimage quality. Thus, one fixed size of coil arrays may not fit allpatients. However, the cost and complexity associated with multipledifferent sized RF coils arrays may preclude the use of different sizedcoils, thus degrading imaging for at least some patients.

BRIEF DESCRIPTION

In one embodiment, a foldable radiofrequency (RF) coil array includes afirst RF coil configured to generate a magnetic field along a firstaxis, the first RF coil having a first surface, a second RF coilconfigured to generate a magnetic field along a second axis, orthogonalto the first axis, the second RF coil having a second surface, and afirst foldable interconnect coupling the first RF coil to the second RFcoil. In an unfolded configuration, the first foldable interconnect isconfigured to couple the first RF coil to the second RF coil with afirst amount of overlap and with the first surface and second surfacefacing a common direction, and in a folded configuration, the firstfoldable interconnect is configured to couple the first RF coil to thesecond RF coil with a second amount of overlap, larger than the firstamount of overlap, and with the first surface in face to face positionwith the second surface.

It should be understood that the brief description above is provided tointroduce in simplified form a selection of concepts that are furtherdescribed in the detailed description. It is not meant to identify keyor essential features of the claimed subject matter, the scope of whichis defined uniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from reading thefollowing description of non-limiting embodiments, with reference to theattached drawings, wherein below:

FIG. 1 shows a diagrammatical representation of an MRI system.

FIGS. 2A and 2B schematically show example radiofrequency (RF) coilgeometries.

FIGS. 3A-4B schematically show an example foldable RF coil array.

FIG. 5 is a block diagram of an embodiment of a receive section of amagnetic resonance imaging (MRI) system including a foldable RF coilarray.

FIG. 6 is a schematic diagram of a portion of the receive section shownin FIG. 5 illustrating an embodiment of a RF receiver coil and anembodiment of a corresponding pre-amplifier of the receive section.

FIG. 7 is a schematic diagram illustrating an embodiment of thepreamplifier shown in FIG. 6.

FIG. 8 is a flow chart illustrating a method for operating an MRI systemhaving a foldable RF coil array.

DETAILED DESCRIPTION

The following description relates to various embodiments of medicalimaging systems. In particular, methods and systems are provided for afoldable radiofrequency (RF) coil array. An example of a magneticresonance imaging (MRI) system that may be used to acquire images isprovided in FIG. 1. The MRI system of FIG. 1 may include one or morefoldable RF coil arrays, as illustrated in FIGS. 3A-4B. The one or morefoldable RF coil arrays may be comprised RF coils having loop andbutterfly geometries, such as the butterfly and loop coils illustratedin FIGS. 2A and 2B. Each RF coil of the foldable RF coil array may bepart of a receive circuit that includes a preamplifier, as shown inFIGS. 5-7. The MR signals received by the foldable RF coil array may beused to reconstruct an image of a region of interest according to themethod illustrated in FIG. 8.

As explained previously, MRI imaging systems use RF coils to acquireimage information of a region of interest of a scanned object. Theresultant image that is generated shows the structure and function ofthe region of interest. Conventional MRI imaging systems may include amultiple-channel coil array having a plurality of coil elements. Thesignals detected by the multiple-channel array coil are processed by acomputer to generate MR images of the object being imaged. Duringoperation, the plurality of coil elements may be inductively orcapacitively decoupled from the other coil elements. Further, theplurality of coil elements may be maintained at a fixed distance fromeach other to maintain desired coil isolation. Thus, the overallcoverage of a conventional RF coil is also fixed. However, RF arrays aretypically sized to be utilized with a patient having an average size.Thus, if the patient is larger or smaller than the average patient, theRF coil array may be too large or too small to properly fit the patient.As a result, in operation the RF coil may not provide sufficientinformation of the patient being imaged. To accommodate larger andsmaller patients, some medical facilities may choose to purchase RFarray coils having different sizes to accommodate patients havingdifferent sizes. However, the costs of such RF coils arrays may limitthe quantity of medical providers capable of expending the additionalfinancial resources required to purchase RF coil arrays having differentsizes.

According to embodiments disclosed herein, a foldable RF coil array isadjustable between a partial overlap configuration (also referred to asan unfolded configuration) and a full overlap configuration (alsoreferred to as a folded configuration). In the partial overlap, unfoldedconfiguration, the RF coil array may be longer or wider in orderaccommodate imaging of relatively large patients or allow imaging of arelatively large portion of anatomy. In the full overlap, foldedconfiguration, the RF coil array may be shorter or narrower toaccommodate imaging of smaller patients or smaller portions of anatomy.An operator may easily adjust the configuration of the foldable RF coilarray, thus preventing the need to keep multiple different sized RF coilarrays on hand, and hence reducing cost.

Turning now to the drawings, and referring first to FIG. 1, a magneticresonance imaging (MRI) system 10 is illustrated diagrammatically asincluding a scanner 12, scanner controller circuitry 14, and systemcontrol circuitry 16. While the MRI system 10 may include any suitableMRI scanner or detector, in the illustrated embodiment the systemincludes a full body scanner comprising an imaging volume 18 into whicha table 20 may be positioned to place a patient 22 in a desired positionfor scanning. The scanner 12 may additionally or alternatively beconfigured to target certain anatomy, such as the head or neck.

The scanner 12 may include a series of associated coils for producingcontrolled magnetic fields, for generating radio frequency (RF)excitation pulses, and for detecting emissions from gyromagneticmaterial within the patient in response to such pulses. In thediagrammatical view of FIG. 1, a main magnet 24 is provided forgenerating a primary magnetic field generally aligned with the imagingvolume 18. A series of gradient coils 26, 28, and 30 are grouped in oneor more gradient coil assemblies for generating controlled magneticgradient fields during examination sequences as described more fullybelow. An RF coil 32 is provided for generating RF pulses for excitingthe gyromagnetic material. Power may be supplied to the scanner 12 inany appropriate manner, as indicated generally at reference number 34.In the embodiment illustrated in FIG. 1, the RF coil 32 may also serveas a receiving coil. Thus, the RF coil 32 may be coupled with receivingand driving circuitry in passive and active modes for receivingemissions from the gyromagnetic material and for applying RF excitationpulses, respectively. Alternatively, various configurations of receivingcoils may be provided separate from RF coil 32. Such coils may includestructures specially adapted for target anatomies, such as head coilassemblies, and so forth. Moreover, receiving coils may be provided inany suitable physical configuration, including phased array coils, andso forth.

In a present configuration, the gradient coils 26, 28, and 30 may beformed of conductive wires, bars, or plates which are wound or cut toform a coil structure which generates a gradient field upon applicationof control pulses. The placement of the coils within the gradient coilassembly may be done in several different orders and with varyingconfigurations, and the scanner 12 may further include complementarygradient coils (in the manner described below) to shield the gradientcoils 26, 28, and 30. Generally, a z-gradient coil 26 may be positionedat an outermost location, and is formed generally as a solenoid-likestructure which has relatively little impact on the RF magnetic field.The gradient coils 28 and 30 may be x-axis and y-axis coilsrespectively.

The gradient coils 26, 28, and 30 of the scanner 12 may be controlled byexternal circuitry to generate desired fields and pulses, and to readsignals from the gyromagnetic material in a controlled manner. When thematerial, typically bound in tissues of the patient, is subjected to theprimary field, individual magnetic moments of the paramagnetic nuclei inthe tissue partially align with the field. While a net magnetic momentis produced in the direction of the polarizing field, the randomlyoriented components of the moment in a perpendicular plane generallycancel one another. During an examination sequence, the RF coil 32 maygenerate an RF pulse at or near the Larmor frequency of the material ofinterest, resulting in a rotation of the net aligned moment to produce anet transverse magnetic moment. This transverse magnetic momentprecesses around the main magnetic field direction, emitting RF signalsthat are detected by the scanner 12 and processed for reconstruction ofthe desired image.

The gradient coils 26, 28, and 30 may serve to generate preciselycontrolled magnetic fields, the strength of which vary over a predefinedfield of view, typically with positive and negative polarity. When eachgradient coil 26, 28, or 30 is energized with known electric current,the resulting magnetic field gradient is superimposed over the primaryfield and produces a desirably linear variation in the axial componentof the magnetic field strength across the field of view. The field mayvary linearly in one direction, but may be homogeneous in the other two.The three gradient coils 26, 28, and 30 have mutually orthogonal axesfor the direction of their variation, enabling a linear field gradientto be imposed in an arbitrary direction with an appropriate combinationof the three gradient coils 26, 28, and 30.

One or more shielding coils, such as shielding coil 31, may be present.The shielding coil 31 comprises turns of a conductive materialconfigured to carry current in an opposite direction as a respectivegradient coil, such as coil 30. Like the primary coil, the shieldingcoil includes a shielding x-coil, a shielding y-coil, and a shieldingz-coil. The shielding coil 31 is configured to create a magnetic fieldthat is substantially the opposite of the field created by the primarycoil for regions outside of the shielding coil 31. For example, theshielding coil 31 is designed to minimize the stray fields from theprimary coil that might otherwise induce eddy currents in otherconducting structures, such as a cryostat (not shown). It is importantto minimize the production of eddy currents in order to prevent thegeneration of time-varying magnetic fields that would otherwisenegatively impact the performance of the MRI system.

The pulsed gradient fields may perform various functions integral to theimaging process. Some of these functions are slice selection, frequencyencoding, and phase encoding. These functions can be applied along thex-, y-, and x-axes of the original coordinate system or along other axesdetermined by combinations of pulsed current applied to the individualfield coils.

The slice select gradient field may determine a slab of tissue oranatomy to be imaged in the patient, and may be applied simultaneouslywith a frequency selective RF pulse to excite a known volume of spinsthat may precess at the same frequency. The slice thickness may bedetermined by the bandwidth of the RF pulse and the gradient strengthacross the field of view.

The frequency encoding gradient, also known as the readout gradient, isusually applied in a direction perpendicular to the slice selectgradient. In general, the frequency encoding gradient is applied beforeand during the formation of the MR echo signal resulting from the RFexcitation. Spins of the gyromagnetic material under the influence ofthis gradient are frequency encoded according to their spatial positionalong the gradient field. By Fourier transformation, acquired signalsmay be analyzed to identify their location in the selected slice byvirtue of the frequency encoding.

Finally, the phase encode gradient is generally applied before thereadout gradient and after the slice select gradient. Localization ofspins in the gyromagnetic material in the phase encode direction isaccomplished by sequentially inducing variations in phase of theprecessing protons of the material using slightly different gradientamplitudes that are sequentially applied during the data acquisitionsequence. The phase encode gradient permits phases differences to becreated among the spins of the material in accordance with theirposition in the phase encode direction.

A great number of variations may be devised for pulse sequencesemploying the exemplary gradient pulse functions described above, aswell as other gradient pulse functions not explicitly described here.Moreover, adaptations in the pulse sequences may be made toappropriately orient the selected slice and the frequency and phaseencoding to excite the desired material and to acquire resulting MRsignals for processing.

The coils of the scanner 12 are controlled by the scanner controlcircuitry 14 to generate the desired magnetic field and radio frequencypulses. In the diagrammatical view of FIG. 1, the control circuitry 14thus includes a control circuit 36 for commanding the pulse sequencesemployed during the examinations, and for processing received signals.The control circuit 36 may include a suitable programmable logic device,such as a CPU or digital signal processor. Further, the control circuit36 may include memory circuitry 38, such as volatile and/or non-volatilememory devices for storing physical and logical axis configurationparameters, examination pulse sequence descriptions, acquired imagedata, programming routines, and so forth, used during the examinationsequences implemented by the scanner 12.

Interface between the control circuit 36 and the coils of the scanner 12may be managed by amplification and control circuitry 40 and bytransmission and receive interface circuitry 42. The amplification andcontrol circuitry 40 includes amplifiers for each gradient field coil26, 28, and 30 to supply drive current in response to control signalsfrom the control circuit 36. The receive interface circuitry 42 includesadditional amplification circuitry for driving the RF coil 32. Moreover,where the RF coil 32 serves both to emit the RF excitation pulses and toreceive MR signals, the receive interface circuitry 42 may include aswitching device for toggling the RF coil between active or transmittingmode, and passive or receiving mode. A power supply, denoted generallyby reference number 34 in FIG. 1, is provided for energizing the primarymagnet 24. Finally, the scanner control circuitry 14 includes interfacecomponents 44 for exchanging configuration and image data with thesystem control circuitry 16.

The system control circuitry 16 may include a wide range of devices forfacilitating interface between an operator or radiologist and thescanner 12 via the scanner control circuitry 14. In the illustratedembodiment, for example, an operator workstation 46 is provided in theform of a computer workstation employing a general purpose orapplication-specific computer. The operator workstation 46 alsotypically includes memory circuitry for storing examination pulsesequence descriptions, examination protocols, user and patient data,image data, both raw and processed, and so forth. The operatorworkstation 46 may further include various interface and peripheraldrivers for receiving and exchanging data with local and remote devices.In the illustrated embodiment, such devices include a monitor 48, aconventional computer keyboard 50, and an alternative input device suchas a mouse 52. A printer 54 is provided for generating hard copy outputof documents and images reconstructed from the acquired data. Inaddition, the system 10 may include various local and remote imageaccess and examination control devices, represented generally byreference number 56 in FIG. 1. Such devices may include picturearchiving and communications systems (PACS), teleradiology systems(telerad), and the like.

FIGS. 2A and 2B show schematic diagrams of example RF coil geometriesthat may be utilized in an RF coil array of the current disclosure. FIG.2A is a diagram 60 illustrating the magnetic field generated bybutterfly coil 62. Butterfly coil 62 may include a figure-eightgeometry, resulting in the illustrated magnetic fields when thebutterfly coil is driven. An overall B1 field 64 is generated, having adirectionality along a first axis (e.g., an x-axis or horizontal axis inthe illustrated diagram). During pulsation of the butterfly coil, therotating RF magnetic field, B1, is applied to tip the magnetization ofthe main magnetic field (B0) generated by the main magnet into a planethat is transverse to B0. When butterfly coil 62 is in a receive mode,rather than generating the illustrated magnetic field due to pulsationfrom a voltage source, the butterfly coil may be sensitive to magneticflux in the illustrated direction.

FIG. 2B is a diagram 70 illustrating the magnetic field generated byloop coil 72. Loop coil 72 may include a single loop geometry, resultingin the illustrated magnetic fields when the loop coil is driven. Anoverall B1 field 74 is generated, having a directionality along a secondaxis (e.g., a y-axis or vertical axis in the illustrated diagram).During pulsation of the loop coil, the rotating RF magnetic field, B1,is applied to tip the magnetization of the main magnetic field (B0)generated by the main magnet into a plane that is transverse to B0. Thebutterfly coil 62 generates a B1 field that is orthogonal to the B1field generated by the loop coil 72. When loop coil 72 is in a receivemode, rather than generating the illustrated magnetic field due topulsation from a voltage source, the loop coil may be sensitive tomagnetic flux in the illustrated direction.

FIGS. 3A and 3B illustrate an example RF coil array 110 that isconfigured to operate in either an unfolded configuration or in a foldedconfiguration. The RF coil array 110 may be a non-limiting example of RFcoil 32 of FIG. 1 and may be configured as a receive-only coil array.

FIG. 3A shows the RF coil array 110 in an unfolded configuration 100. RFcoil array 110 is comprised of a plurality of individual RF coilsarranged in a row. The RF coils may be surface coil elements comprisedof a generally flexible, flat, conductive material, for example,tin-plated copper. The RF coils may be mounted on a flexible substrate(not shown in FIG. 3A). The flexible substrate may be fabricated from asubstantially RF transparent material. For example, the substrate may befabricated from a cloth material or any other suitable material that isflexible to enable the RF coils to be repositioned as described herein.

As shown, RF coil array 110 is comprised of five individual RF coils,first RF coil 112, second RF coil 114, third RF coil 116, fourth RF coil118, and fifth RF coil 120, although more or fewer coils could beincluded without departing from the scope of the disclosure. First RFcoil 112, third RF coil 116, and fifth RF coil 120 may each have a loopcoil geometry, and thus may generate a B1 magnetic field along a firstaxis, which in one example may be parallel to the longitudinal axis ofthe coil array. Second RF coil 114 and fourth RF coil 118 may each havea butterfly coil geometry, and thus may generate a B1 magnetic fieldalong a second axis, orthogonal to the first axis. In one example, thesecond axis may be perpendicular to the longitudinal axis of the coilarray.

Each of the first RF coil 112, second RF coil 114, third RF coil 116,fourth RF coil 118, and fifth RF coil 120 may have a first surfacefacing a common direction (e.g., up/outward) in the unfoldedconfiguration and a second surface, opposite the first surface, thatfaces the flexible substrate in the unfolded configuration. Further,each butterfly coil may have a cross-over region 115 located in thecenter of the butterfly coil. In the unfolded configuration, the firstRF coil 112 does not overlap the cross-over region 115. Likewise, thefifth RF coil 120 does not overlap the cross-over region of fourth RFcoil 118.

First RF coil 112 may be coupled to second RF coil 114 via a firstfoldable interconnect 122. The first foldable interconnect 122 mayinclude a hinge or other mechanism to allow manual adjustment of aposition of first RF coil 112 relative to second RF coil 114. Likewise,fifth RF coil 120 may be coupled to fourth RF coil 118 via a secondfoldable interconnect 124. The second foldable interconnect 124 mayinclude a hinge or other mechanism to allow manual adjustment of aposition of fifth RF coil 120 relative to fourth RF coil 118. In otherexamples, the foldable interconnects may be dispensed with, and rails orother coupling elements may be provided to facilitate sliding of thefirst and fifth RF coils.

RF coil array 110, while in the unfolded configuration 100, may have anoverall length L1 that is defined from a first, outermost edge of firstRF coil 112 to a second, outermost edge of fifth RF coil 120. The RFcoils in RF coil array 110 may be arranged symmetrically, such that adistance D1 between first RF coil 112 and third RF coil 116 is equal toa distance between third RF coil 116 and fifth RF coil 120. As shown,distance D1 includes a distance from a center-point of first RF coil 112to a center-point of third RF coil 116. A second distance D2 may includea distance from a center-point of second RF coil to a center-point offourth RF coil 118.

As shown, each RF coil of RF coil array 110 overlaps at least one otherRF coil. For example, first RF coil 112 overlaps second RF coil 114,second RF coil 114 overlaps both first RF coil 112 and third RF coil116, third RF coil overlaps both second RF coil 114 and fourth RF coil118, fourth RF coil overlaps both third RF coil 116 and fifth RF coil120, and fifth RF coil overlaps fourth RF coil 118. Each RF coil mayoverlap an adjacent RF coil by a suitable amount, based on desiredimaging parameters and system configuration. However, the amount ofoverlap may be relatively small, such that adjacent RF coils do notoverlap each other by more than 50% of an RF coil's respective width.For example, first RF coil 112 may overlap second RF coil by an amountcorresponding to 10% of the overall width of first RF coil 112. Asshown, first RF coil 112 has a width W1, and an overlap width 111 maycomprise 10% of W1.

In some examples, one or more RF coils may be arranged such that nooverlap exits between that RF coil and an adjacent RF coil; in anexample, no overlap may exist between any of the RF coils of RF coilarray 110 when in the unfolded configuration.

FIG. 3B shows RF coil array 110 in a folded configuration 150. To moveRF coil array 110 from the unfolded to the folded configuration, anoperator may manually move first RF coil 112 from an unfolded firstposition to a folded second position, causing first foldableinterconnect 122 to swing, fold, or otherwise hinge in order tofacilitate movement of first RF coil 112. When in the folded secondposition, first RF coil 112 may fully overlap with second RF coil 114,e.g., 100% of the width W1 of first RF coil 112 may overlap with secondRF coil 114, as shown by overlap width 113 being equal to W1. As such,first RF coil 112 may lie on top of second RF coil 114. In the foldedconfiguration, the first surface of the first RF coil 112 may in a faceto face position with the first surface of the second RF coil 114. Inthe face to face to position, the first surfaces may face each other,and may be in full or partial contact. In other examples, the firstsurfaces may face each other but may not make contact with each other.In this way, the first RF coil 112 may be rotated such that its firstsurface is flipped to face the flexible substrate. The first RF coil 112may fully overlap the cross-over region 115 of the second RF coil 114.Additionally, each of the first RF coil 112 and second RF coil 114 mayhave a center point, and when folded, the center points may align.Further, each of the first RF coil 112 and second RF coil 114 may have acentral longitudinal axis and a central lateral axis that each passthrough a respective center point, and when folded, the centrallongitudinal axes of the two coils may align and the central lateralaxes of the two coils may align. The net magnetic flux from thebutterfly or loop coil is zero if the butterfly coil and the loop coilshare the same axis since the incoming flux is equal to outgoing flux(as illustrated in the magnetic flux diagrams of FIGS. 2A and 2B), whichresults in zero coupling between a butterfly and a loop coil.

Likewise, an operator may move fifth RF coil 120 from an unfolded firstposition to a folded second position, causing second foldableinterconnect 124 to swing, fold, or otherwise hinge in order tofacilitate movement of fifth RF coil 120. When in the folded secondposition, fifth RF coil 120 may fully overlap with fourth RF coil 118,e.g., 100% of the width of fifth RF coil 120 may overlap with fourth RFcoil 118. As such, fifth RF coil 120 may lie on top of fourth RF coil118. In the folded configuration, the first surface of the fifth RF coil120 may in a face to face position with the first surface of the fourthRF coil 118. In the face to face to position, the first surfaces mayface each other, and may be in full or partial contact. In otherexamples, the first surfaces may face each other but may not makecontact with each other. In this way, the fifth RF coil 120 may berotated such that its first surface is flipped to face the flexiblesubstrate. Additionally, each of the fourth RF coil 118 and fifth RFcoil 120 may have a center point, and when folded, the center points mayalign. Further, each of the fourth RF coil 118 and fifth RF coil 120 mayhave a central longitudinal axis and a central lateral axis that eachpass through a respective center point, and when folded, the centrallongitudinal axes of the two coils may align and the central lateralaxes of the two coils may align.

Once RF coil array 110 is in the folded configuration, RF coil array 110may have a second length L2 defined from a first outermost edge ofsecond RF coil 114 to a second outermost edge of fourth RF coil 118. Thesecond length L2 may be shorter than the first length L1 of the RF coilarray 110 in the unfolded configuration. Further, the folded RF coilarray 110 may have a third distance D3 between first RF coil 112 andthird RF coil 116 that is smaller than first distance D1. Likewise, thedistance between the fifth RF coil 120 and third RF coil 116 may besmaller than the first distance D1. However, the second distance D2 maybe maintained constant in the unfolded and folded configurations. In oneexample, the first distance D1 may be equal to or greater than thesecond distance D2 and the third distance D3 may be less than the seconddistance D2.

As explained previously, RF coil arrays may be designed to reduce and/orcompensate for the mutual inductance (also referred to as coupling)between adjacent RF coils. However, placing one RF coil on top ofanother RF coil (e.g., fully overlapping the coils) may induce a highamount of coupling between the RF coils, which may degrade imaging.Thus, to minimize RF coil coupling when in the folded configuration, thefirst RF coil 112 may generate a magnetic field that is orthogonal tothe magnetic field generated by the second RF coil 114. As one example,first RF coil 112 may be of a loop configuration while second RF coil114 may be of a butterfly configuration. As such, when first RF coil 112is brought into full overlap with second RF coil 114, essentially nocoupling between the two RF coils occurs. While the quadrature coilgeometry described herein (e.g., a loop and a butterfly coil) may resultin isolated RF coils, other configurations may be possible. For example,first RF coil 112 may be rotated relative to second RF coil 114 in orderto generate a magnetic field that is orthogonal to the magnetic fieldgenerated by the second RF coil.

Further, as explained above, in the folded configuration, the distancebetween adjacent loop coils (e.g., between first RF coil 112 and thirdRF coil 116, and between third RF coil 116 and fifth RF coil 120) isgreatly reduced relative to the unfolded configuration. For example, asshown in FIGS. 3A and 3B, the edges of first RF coil 112 and third RFcoil 116 may be brought into (or close to) contact when in the foldedconfiguration, but may be spaced well apart in the unfoldedconfiguration. This close positioning of the loop coils (which generatemagnetic fields along the same axis) may cause coupling of the RF coils,degrading imaging. To counteract this coupling, high source impedancepreamplifiers may be included in the RF coil circuit, which will beexplained in more detail below with respect to FIGS. 5-7.

Thus, an RF coil array comprising two or more RF coils may be configuredto have an unfolded configuration, where overlap between two adjacent RFcoils is at a minimum amount, and a folded configuration, where overlapbetween at least two adjacent RF coils is a maximum amount. By providinga foldable RF coil array, an appropriate size RF coil array may beselected for a given patient size and/or type of anatomy being imaged.For example, the unfolded configuration may be selected for a relativelylarge (e.g., tall) patient, while the folded configuration may beselected for a relatively small (e.g., short) patient. In anotherexample, the unfolded configuration may be selected for imaging a torso,while the folded configuration may be selected for imaging a head. Inthis way, a cost-effective mechanism may be provided for imaging a widevariety of patient types and anatomy.

Additionally, the folded configuration may provide increasedsignal-to-noise ratio (SNR) relative to the unfolded configuration, andalso relative to standard, four loop RF coil arrays. For example, astandard four-loop RF coil array having a length equal to the foldedconfiguration of the RF coil array 110 of FIG. 3B may have an SNR of 124for a given set of imaging conditions, while the folded configurationdescribed herein may have an SNR at the same imaging conditions of 153,resulting in an increased SNR of 23%. The unfolded configuration mayresult in an SNR of 135 for the given imaging conditions, an increase of9% over the standard four-loop RF coil array. Thus, by providing twosets of fully overlapping RF coils, the folded configuration of the RFcoil array described herein may increase SNR, even relative to an RFcoil array of equal length.

Further, the unfolded configuration of the RF coil array 110 illustratedin FIG. 3A may improve parallel imaging relative to the foldedconfiguration, due to the spacing between the coils in the unfoldedconfiguration. Parallel imaging results in scan time reduction,resolution enhancement, artifact suppression, and, even attenuation ofnoise. In a general sense, parallel imaging utilizes the difference insensitivities between individual coils of a receiver array to reduce thenumber of gradient encoding steps required for imaging. Thus, inparallel MRI, an array of receiver coils with different sensitivities isused to receive the signal in parallel, facilitating combination ofthese obtained signals to reconstruct the full image.

There are several parallel MRI approaches, including SMASH (SiMultaneousAcquisition of Spatial Harmonics) and SENSE (SENSitivity Encoding). Forpulse sequences that execute a rectilinear trajectory in k space, thesetechniques reduce the number of phase encoding steps in order to reduceimaging time, and then use array sensitivity information to make up forthe loss of spatial information.

In order to make the coils in the array sufficiently spatially distinct,and thus improve their signal-to-noise ratio (SNR) for acceleratedimaging, it is common to leave gaps between neighboring coils in thearray. This, however, has the drawback of increasing the couplingbetween coils, which can in turn degrade performance. To overcome thislimitation, the coils in the array may be overlapped by an amount thatminimizes the mutual inductance between neighboring coils. When such anarray is employed for parallel imaging, the SNR decreases, because thegeometry factor of the array has increased.

Thus, RF coil array design may include a trade-off between allowingparallel imaging by providing spatially distinct coils, while reducingcoupling between coils and providing adequate SNR. However, the RF coilarray of the current disclosure may reduce these trade-offs by providingdistinct coil array geometries in different configurations. For example,an operator may choose to increase SNR by utilizing the coil array inthe folded configuration or the operator may choose to perform parallelimaging by utilizing the coil array in the unfolded configuration.

In some examples, the RF coil array 110 described with respect to FIGS.3A and 3B may form part of a larger RF coil array. In such an example,the RF coil array 110 may comprise an RF coil array element, andmultiple RF coil array elements may be included in the larger RF coilarray. An example of a larger RF coil array configured to be used in afolded or unfolded configuration is illustrated in FIGS. 4A and 4B.

FIG. 4A shows a foldable RF coil array 400 in an unfolded configuration,comprising a plurality of RF coil array elements. Each RF coil arrayelement may comprise a row of RF coil array 400. As shown, RF coil array400 includes a first RF coil array element 408, a second RF coil arrayelement 410, a third RF coil array element 412, and a fourth RF coilarray element 414, although more or fewer RF coil array elements may beincluded without departing from the scope of this disclosure. Each RFcoil array element may be comprised of alternating loop and butterfly RFcoils, as explained above with respect to FIG. 3A. Thus, RF coil array400 may include a first column of loop coils, a second column ofbutterfly coils, a third column of loop coils, a fourth column ofbutterfly coils, and a fifth column of loop coils.

Each RF coil of the RF coil array may be mounted, fixed, or otherwisecoupled to a flexible substrate 402, which may be comprised of fabric,foam, or other suitable material. The flexible substrate 402 may includea first foldable seam 404 and a second foldable seam 406. The foldableseams may facilitate folding of the RF coil array. In this way, eachfirst RF coil of each RF coil array element may simultaneously be foldedover each second RF coil of each RF coil array element. Likewise, eachfifth RF coil of each RF coil array element may simultaneously be foldedover each fourth RF coil of each RF coil array element. The foldable RFcoil array 400 in a folded configuration 450 is shown in FIG. 4B.

FIG. 5 is a block diagram of an embodiment of a receive section of amagnetic resonance imaging (MRI) system including an RF coil array 201.RF coil array 201 is a non-limiting example of RF coil array 32, RF coilarray 110, and/or RF coil array 400 of FIGS. 1, 3A, and 4A,respectively. It should be realized that although the receive section isdescribed with respect to the RF coil array 201, the receive section maybe utilized with any of the RF coil arrays described herein.

As illustrated in FIG. 5, various embodiments may be implemented inconnection with a receive section 200 of an MRI system. The receivesection 200 is configured to acquire MR data using an RF coil array 201such as the RF coil arrays 32, 110, and 400 described herein. Asdiscussed, the RF coil array 201 that includes a plurality of RFreceiver coils 202 (illustrated as a single block element in FIG. 5 forsimplicity). For example, the RF coil array 201 may include a pluralityof loop and/or butterfly elements that form the RF receiver coils 202.The RF receiver coils 202 are configured to detect MR signals. It shouldbe noted that a subset of the RF receiver coils 202, for exampleadjacent loop and butterfly elements, may be fully or partiallyoverlapped as described herein. The RF receiver coils 202 are alsoisolated from each other using preamplifiers 210 that also amplifyreceived MR signals from the RF receiver coils 202. In the exemplaryembodiment, the RF coil array 201 is a dedicated receive only coilarray. Alternatively, the RF coil array 201 is a switchable array, suchas a switchable transmit/receive (T/R) phased array coil. Portionsand/or an entirety of the receive section 200 may be referred to hereinas a “system”.

Thus, the RF coil array 201 forms part of the multi-channel receivesection 200 connected to an MRI system. The receive section 200 includesa plurality of channels (Rcvr 1 . . . Rcvr N), for example, twentychannels. However, it should be noted that more or less channels may beprovided based on the quantity of RF coils 202 utilized to form the RFcoil array 201. In the exemplary embodiment, the RF coil array 201 isconnected to the multi-channel receive section 200 having amulti-channel system interface 220 (e.g., a 1.5T System Interface), witha separate receive channel 222 connected to each one of the plurality ofthe RF receiver coils 202 (e.g., sixteen channels connected to a four byfour coil array).

The system interface 220 may include a plurality of bias control lines224 (illustrated as two lines) to control the switching of decouplingcircuits (not shown), which may be controlled, for example, using a coilconfiguration file stored in the MRI system and/or based on a userinput. For example, based on a user input, a particular coilconfiguration file may be selected to control the RF coil array 201configured as a T/R phased array coil in a particular imaging mode(e.g., user control of mode of operation using controls on an MRIscanner). An RF IN control line 226 also may be provided in connectionwith, for example, a combiner (not shown) to control a transmit coilarray.

FIG. 6 is a schematic diagram of a portion of the receive section 200illustrating an embodiment of one of the RF receiver coils 202 and anembodiment of a corresponding pre-amplifier 210. In the exemplaryembodiment, the preamplifier 210 has a relatively low input impedance.For example, in some embodiments, a “relatively low” input impedance ofthe preamplifier 210 is less than approximately 5 ohms at resonancefrequency. The input impedance of the preamplifier 210 is defined by aninductor 230, which is shown in FIG. 7. Referring again to FIG. 6, theinput impedance of the preamplifier 210 is represented by Z_(IN). Insome embodiments, the preamplifier 210 has an input impedance of betweenapproximately 1 ohm and approximately 3 ohms at resonance frequency.Moreover, in some embodiments, the preamplifier 210 has an inputimpedance of approximately 2 ohms at resonance frequency. It should benoted that for purposes of illustration, all of the capacitors areconsidered lossless and the inductors are represented with a seriesresistance. The input impedance of the preamplifier 210 may be referredto herein as a “preamplifier input impedance.”

The RF receiver coil 202 includes an RLC resonant circuit formed from aresistor 250, an inductor 252, and a capacitor 254. The RF receiver coil202 is also connected in series to an impedance transformer 256. Morespecifically, the impedance transformer 256 is electrically connectedbetween the RF receiver coil 202 and the preamplifier 210. The impedancetransformer 256 forms an impedance matching network between the RFreceiver coil 202 and the preamplifier 210. The impedance transformer256 is configured to transform a coil impedance of the RF receiver coil202 into a source impedance of the preamplifier 210. The sourceimpedance of the preamplifier 210 is represented in FIG. 6 by Z_(OUT).The coil impedance of the RF receiver coil 202 may have any value, whichmay be dependent on coil loading, coil size, field strength, and/or thelike. Examples of the coil impedance of the RF receiver coil 202include, but are not limited to, between approximately 2 ohms andapproximately 10 ohms at 1.5T field strength, and/or the like.

In one exemplary embodiment, the impedance transformer 256 includes alattice-type balun. More specifically, the impedance transformer 256includes two inductors 260 and 262 and two capacitors 264 and 266. Theinductor 260 is connected in series to the capacitor 264, while theinductor 262 is connected in series with the capacitor 266. The inductor260 and the capacitor 264 are connected in parallel to the inductor 262and the capacitor 266. In the exemplary embodiment, the arrangement ofthe lattice-type balun impedance transformer 256 produces a +/−90° phaseshift. Each of the inductors 260 and 262 may be referred to herein as a“first” and/or a “second” inductor. The capacitors 264 and 266 may bereferred to herein as a “first” and/or a “second” capacitor.

The impedance transformer 256 is configured to transform the coilimpedance of the RF receiver coil 202 into a relatively high sourceimpedance Z_(OUT). For example, in some embodiments, a “relatively high”source impedance Z_(OUT) is at least approximately 100 ohms.Accordingly, in the exemplary embodiment, the impedance transformer 256is configured to transform the coil impedance of the RF receiver coil202 into a source impedance Z_(OUT) of at least approximately 100 ohms.In some embodiments, the impedance transformer 256 is configured totransform the coil impedance of the RF receiver coil 202 into a sourceimpedance Z_(OUT) of at least approximately 300 ohms, at leastapproximately 400 ohms, or at least approximately 500 ohms. Exemplaryvalues for the inductors 260 and 262 include, but are not limited to,approximately 123.5 nH. Exemplary values for the capacitors 264 and 266include, but are not limited to, approximately 51 pF.

The impedance transformer 256 also provides a blocking impedance to theRF receiver coil 202. Transformation of the coil impedance of the RFreceiver coil 202 to a relative high source impedance Z_(OUT) may enablethe impedance transformer 256 to provide a higher blocking impedance tothe RF receiver coil 202. Because the relatively high source impedanceZ_(OUT) of the preamplifier 210 is greater than, for example, theconventional value of approximately 50 ohms, the reactance X of theinductors 260 and 262 and the capacitors 264 and 266 of the impedancetransformer 256 are increased. For example, the reactance XC of each ofthe capacitors 264 and 266and the reactance XL of each of the inductors260 and 262 can be defined by the equation: XC=XL=√(R1×R2); where R1 isthe coil impedance and R2 is the source impedance Z_(OUT). Because theinput impedance Z_(IN) of the preamplifier 210 is relatively low, theimpedance transformer 256 forms a parallel resonance circuit thatresults in a higher impedance at an output 270 of the RF receiver coil202. As the reactances XC and XL increase, the blocking impedanceincreases because the blocking impedance is directly proportional to thevalues of XC and XL. The higher blocking impedance suppresses anincreased amount of RF current along the RF receiver coil 202, which mayultimately result in a higher SNR ratio because of fewer interactionsbetween RF receiver coils 202 and/or less correlated noise. Exemplaryvalues for such higher blocking impedances include, for example, ablocking impedance of at least 500 ohms, and at least 1000 ohms.

The impedance transformer 256 is not limited to a lattice-type balunstructure for transforming the coil impedance of the RF receiver coil202 into a relatively high source impedance. Rather, any components andarrangement of the connections therebetween may be used to transform thecoil impedance of the RF receiver coil 202 into a relatively high sourceimpedance, such as, but not limited to, other types of equivalent phaseshift baluns, and/or the like.

FIG. 7 is a schematic diagram illustrating an embodiment of thepreamplifier 210 shown in FIG. 5. The preamplifier 210 is configured toaccommodate the relatively high source impedance Z_(OUT) while providingthe relatively low input impedance Z_(IN). The input impedance Z_(IN) ofthe preamplifier 210 is defined by the inductor 230 of the preamplifier210. The preamplifier 210 includes an amplifier 280 that receives MRsignals from the corresponding RF receiver coil 202 and amplifies thereceived MR signals. An input circuit 282 is electrically connected tothe amplifier 280. The input circuit 282 is electrically connected tothe output 270 (shown in FIG. 6) of the corresponding RF receiver coil202, via the impedance transformer 256 (shown in FIG. 6). The inputcircuit 282 is configured to transmit the MR signals from thecorresponding RF receiver coil 202 to the amplifier 280.

The input circuit 282 includes an impedance transformer 284, whichincludes a capacitor 286 and the inductor 230. The input circuit 282also includes a field effect transistor (FET) 288 that is electricallyconnected between the impedance transformer 284 and the amplifier 280,for example as shown in FIG. 6. The impedance transformer 284 iselectrically connected between the amplifier 280 and the correspondingRF receiver coil 202.

In the exemplary embodiment, the FET 288 has a relatively large noisecircle, which may be centered in the Smith Chart, for the FET 288 toyield a relatively low noise figure. In other words, the FET 288 iscapable of providing a relatively low noise figure over a relativelybroad range of source impedance Z_(OUT). For example, in someembodiments, a “relatively large” size of the noise circle of the FET288 is at least approximately 0.3 decibels. In some embodiments, thenoise circle of the FET 288 has a size of at least approximately 0.6decibels. The size of the noise circle of the FET 288 is dependent onthe noise resistance RN of the FET 288. The FET 288 may have any valueof noise resistance RN that provides a noise circle having a size of atleast 0.3 decibels, such as, but not limited to, less than approximately0.03 ohms, equal to or less than approximately 0.02 ohms, and/or thelike. The location of the noise circle of the FET 288 within the SmithChart is dependent on the optimum reflection coefficient of the FET 288.For example, the noise circle of the FET 288 may be located closer tothe center of the Smith Chart (i.e., closer to being concentric) whenthe optimum reflection coefficient of the FET 288 is less thanapproximately 100 ohms. In some embodiments, the noise circle of the FET288 is centered within the Smith Chart (i.e., concentric with the SmithChart). In some embodiments, and for example, the FET 288 has an optimumreflection coefficient of less than approximately 100 ohms. In someembodiments, and for example, the FET 288 has an optimum reflectioncoefficient of between approximately 40 ohms and approximately 60 ohms,for example approximately 50 ohms.

Turning to FIG. 8, a method 800 for operating a magnetic resonanceimaging (MRI) system is provided. Method 800 may be at least partiallycarried out by a processor, such as a processor of control circuit 36 ofFIG. 1, in order to perform imaging using a foldable RF coil array, suchas RF coil array 110 of FIGS. 3A and 3B and/or RF coil array 400 of FIG.4A.

At 802, method 800 includes determining an RF coil array configuration.The foldable RF coil array may be adjustable to an unfolded (e.g.,partial overlap) configuration, as described above with respect to FIG.3A, or to a folded (e.g., full overlap) configuration, as describedabove with respect to FIG. 3B. The RF coil array configuration may beselected based on patient or anatomy size and/or desired imagingparameters, as indicated at 804. In one example, if a relatively largepatient is being imaged, or if a relatively large portion of anatomy isbeing imaged, the unfolded configuration may be selected, while if arelatively small patient or small portion of anatomy is being imaged,the folded configuration may be selected. In another example, if a highSNR is desired, the folded configuration may be selected, while ifparallel imaging is desired, the unfolded configuration may be selected.The selection of the RF coil array configuration may be made by anoperator in one example. In another example, the selection may be madeautomatically by the processor based on user input of desired imagingparameters, anatomy being imaged, and/or determination of patient size.

At 806, method 800 includes determining if the RF coil array is in afolded configuration. In one example, the determination may be madebased on user input. In another example, the determination may beassumed based on the type of imaging being performed, e.g., torso vs.head imaging, parallel vs. non-parallel imaging, etc.

If the RF coil is in the folded configuration, method 800 proceeds to808 to pulse a transmitter RF coil array. In one example, the foldableRF coil array may be a transmit/receive array, and thus pulsing thetransmitter RF coil array may include pulsing the foldable RF coil array(in the folded configuration). In another example, the foldable RF coilarray may be a receive only array, and thus pulsing the transmitter RFcoil array may include pulsing a separate array.

The magnetic flux that results from the pulsing of the transmitter RFcoil array induces a current in the receive RF coil array (e.g., in thefoldable RF coil array), which current is subsequently amplified andprocessed to reconstruct an image of the patient/region of interest.Thus, at 810, method 800 includes receiving the resultant magnetic fluxby the foldable RF coil array in the folded configuration. At 812, theRF coil impedance is transformed into high source input impedance via apreamplifier, as explained above with respect to FIGS. 5-7. Each RF coilof the foldable RF coil array may be coupled to a respectivepre-amplifier as described above, and thus the impedance of each RF coilmay be transformed. In doing so, coupling between adjacent RF coils maybe reduced.

At 814, the MR signals from all the RF coils from the foldable RF coilarray are combined, and the combined signal is used to reconstruct animage of the region of interest, as indicated at 816. Method 800 thenreturns.

Returning to 806, if is determined that the foldable RF coil array isnot in the folded configuration, the foldable RF coil array is hence inthe unfolded configuration and method 800 proceeds to 818 to pulse thetransmitter RF coil array (which may be the foldable RF coil array or aseparate array, as explained above). The resultant magnetic flux isreceived by the foldable RF coil array in the unfolded configuration, asindicated at 820. At 822, the RF coil impedance from each RF coil of thefoldable RF coil array is transformed into high source input impedancevia a respective preamplifier, as explained above. At 824, if parallelimaging is indicated (e.g., based on user input), each signal from eachRF coil is processed separately, and at 826, an image is reconstructedfrom the received, separate MR signals. Method 800 then returns.

A technical effect of the disclosure is providing an adjustable RF coilarray to accommodate imaging of a variety of patient sizes whileincreasing SNR.

In one embodiment, a foldable radiofrequency (RF) coil array includes afirst RF coil configured to generate a magnetic field along a firstaxis, the first RF coil having a first surface; a second RF coilconfigured to generate a magnetic field along a second axis, orthogonalto the first axis, the second RF coil having a second surface; and afirst foldable interconnect coupling the first RF coil to the second RFcoil, where in an unfolded configuration, the first foldableinterconnect is configured to couple the first RF coil to the second RFcoil with a first amount of overlap and with the first surface andsecond surface facing a common direction, and in a folded configuration,the first foldable interconnect is configured to couple the first RFcoil to the second RF coil with a second amount of overlap, larger thanthe first amount of overlap, and with the first surface in face to faceposition with the second surface. The first amount of overlap mayinclude 10% or less of a width the first RF coil overlapping with thesecond RF coil, and the second amount of overlap may include 90% orgreater of the width the first RF coil overlapping with the second RFcoil. In one example, the second amount of overlap may include fulloverlap where 100% of the width of the first RF coil overlaps the secondRF coil. The first foldable interconnect may include a hinge configuredto rotate to allow the first RF coil to move from the unfoldedconfiguration to the folded configuration. In an example, the first RFcoil is a loop RF coil and the second RF coil is a butterfly RF coil.The butterfly RF coil may include a cross-over region, wherein in theunfolded configuration, the loop RF coil does not overlap the cross-overregion, and wherein in the folded configuration, the loop RF coiloverlaps the cross-over region.

The foldable RF coil array may further include a third RF coilconfigured to generate a magnetic field along the first axis; a fourthRF coil configured to generate a magnetic field along the second axis; afifth RF coil configured to generate a magnetic field along the firstaxis; and a second foldable interconnect coupling the fifth RF coil tothe fourth RF coil. The first RF coil may be separated from the third RFcoil by a first distance in the unfolded configuration and may beseparated from the third RF coil by a second distance in the foldedconfiguration, the second distance less than the first distance, and thesecond RF coil and fourth RF coil may be separated by a third distancein both the folded configuration and the unfolded configuration. In theunfolded configuration, the RF coil array may have a first lengthdefined by a first outermost edge of the first RF coil and a secondoutermost edge of the fifth RF coil, and in the folded configuration,the RF coil array may have a second length defined by a first outermostedge of the second RF coil and a second outermost edge of the fourth RFcoil. In the unfolded configuration, the second RF coil, third RF coil,and fourth RF may each be positioned intermediate the first RF coil andthe fifth RF coil, and in the folded configuration, the first RF coil,third RF coil, and fifth RF coil may each be positioned intermediate thesecond RF coil and fourth RF coil.

The first RF coil, second RF coil, and first foldable interconnect maydefine a first coil element, and the RF coil array may further includeone or more additional coil elements each comprising a respective firstRF coil, second RF coil, and first foldable interconnect, the first coilelement and one or more additional coil elements arranged into rows. Thefirst coil element and one or more additional coil elements may bemounted on a flexible substrate, the flexible substrate having afoldable seam aligned with each first foldable interconnect of each coilelement.

The RF coil array may further include a preamplifier coupled to thefirst RF coil, the preamplifier including an amplifier and an impedancetransformer to transform a coil impedance of the first RF coil to asource impedance of at least approximately 100 ohms.

In an embodiment, a magnetic resonance imaging (MRI) system, comprises agantry having a bore extending therethrough; and a radio frequency (RF)coil array configured to be inserted into the bore, the RF coil arraycomprising: an RF coil flexible substrate; and a plurality of RF coilscoupled to the RF coil flexible substrate, the RF coil flexiblesubstrate configured to enable the plurality of RF coils to bepositioned in a partial overlap configuration and repositioned to a fulloverlap configuration. The plurality of RF coils may include a first setof loop coils arranged in a first column and a second set of butterflycoils arranged in a second, adjacent column. In the partial overlapconfiguration, the first set of loop coils and second set of butterflycoils may overlap by less than threshold amount, and in full overlapconfiguration, the first set of loop coils and second set of butterflycoils may fully overlap. The MRI system may further include a controlcircuit configured to reconstruct an image based on signals received bythe plurality of RF coils. The control circuit may be configured toreconstruct the image according to a parallel imaging protocol when theRF coil array is the partial overlap configuration.

In an embodiment, a method includes, during a first condition, receivinga first plurality of magnetic resonance (MR) signals via aradiofrequency (RF) coil array arranged in a partial overlapconfiguration, and generating one or more images of a first region ofinterest based on the received first plurality of MR signals; and duringa second condition, receiving a second plurality of MR signals via theRF coil array arranged in a full overlap configuration, and generatingone or more images of a second region of interest based on the receivedsecond plurality of MR signals. The first condition may comprise thefirst region of interest being larger than a threshold, and the secondcondition may comprise the second region of interest being smaller thanthe threshold. The RF coil array may include a first set of loop coilsarranged in a first column and a second set of butterfly coils arrangedin a second, adjacent column. In the partial overlap configuration, thefirst set of loop coils and second set of butterfly coils may overlap byless than threshold amount, and in the full overlap configuration, thefirst set of loop coils and second set of butterfly coils may fullyoverlap. In one example, generating one or more images of the secondregion of interest based on the received second plurality of MR signalsincludes generating one or more images of the second region of interestusing parallel imaging.

In any of the above described embodiments, an amplifier may beconfigured to receive at least one magnetic resonance (MR) signal froman RF coil and configured to generate an amplified MR signal; and aninput circuit may be electrically connected to the amplifier, the inputcircuit being configured to be electrically connected to an output ofthe RF coil for transmitting the at least one MR signal from the RF coilto the amplifier, the input circuit comprising an impedance transformerand a field effect transistor (FET), the FET being electricallyconnected between the impedance transformer and the amplifier, the FEThaving an FET impedance, the impedance transformer being configured totransform a source impedance of at least approximately 100 ohms, theimpedance transformer being further configured to transform the FETimpedance into a preamplifier input impedance of less than approximately5 ohms. The impedance transformer may be configured to transform thesource impedance into an impedance that is within a 0.3 dB noise circleof the FET.

As used herein, an element or step recited in the singular and proceededwith the word “a” or “an” should be understood as not excluding pluralof said elements or steps, unless such exclusion is explicitly stated.Furthermore, references to “one embodiment” of the present invention arenot intended to be interpreted as excluding the existence of additionalembodiments that also incorporate the recited features. Moreover, unlessexplicitly stated to the contrary, embodiments “comprising,”“including,” or “having” an element or a plurality of elements having aparticular property may include additional such elements not having thatproperty. The terms “including” and “in which” are used as theplain-language equivalents of the respective terms “comprising” and“wherein.” Moreover, the terms “first,” “second,” and “third,” etc. areused merely as labels, and are not intended to impose numericalrequirements or a particular positional order on their objects.

This written description uses examples to disclose the invention,including the best mode, and also to enable a person of ordinary skillin the relevant art to practice the invention, including making andusing any devices or systems and performing any incorporated methods.The patentable scope of the invention is defined by the claims, and mayinclude other examples that occur to those of ordinary skill in the art.Such other examples are intended to be within the scope of the claims ifthey have structural elements that do not differ from the literallanguage of the claims, or if they include equivalent structuralelements with insubstantial differences from the literal languages ofthe claims.

The invention claimed is:
 1. A magnetic resonance imaging (MRI) system, comprising: a gantry having a bore extending therethrough; and a foldable radio frequency (RF) coil array configured to be inserted into the bore, the foldable RF coil array comprising: an RF coil flexible substrate; and a plurality of RF coils coupled to the RF coil flexible substrate, the RF coil flexible substrate configured to enable the plurality of RF coils to be positioned in an unfolded configuration and repositioned to a folded configuration, wherein the RF coil flexible substrate includes one or more foldable seams to facilitate folding of the foldable RF coil array.
 2. The MRI system of claim 1, wherein the plurality of RF coils includes a first set of loop coils arranged in a first column and a second set of butterfly coils arranged in a second, adjacent column, wherein in the unfolded configuration, the first set of loop coils and the second set of butterfly coils overlap by less than a threshold amount, and in the folded configuration, the first set of loop coils and the second set of butterfly coils fully overlap.
 3. The MRI system of claim 1, further comprising a control circuit configured to reconstruct an image based on signals received by the plurality of RF coils.
 4. The MRI system of claim 3, wherein the control circuit is configured to reconstruct the image according to a parallel imaging protocol when the foldable RF coil array is in the unfolded configuration.
 5. A method, comprising: during a first condition, receiving a first plurality of magnetic resonance (MR) signals via a foldable radiofrequency (RF) coil array arranged in an unfolded configuration, and generating one or more images of a first region of interest based on the received first plurality of MR signals; and during a second condition, receiving a second plurality of MR signals via the foldable RF coil array arranged in a folded configuration, and generating one or more images of a second region of interest based on the received second plurality of MR signals, wherein the foldable RF coil array comprises an RF coil flexible substrate that includes one or more foldable seams to facilitate folding of the foldable RF coil array.
 6. The method of claim 5, wherein the first condition comprises the first region of interest being larger than a threshold, and wherein the second condition comprises the second region of interest being smaller than the threshold.
 7. The method of claim 5, wherein the foldable RF coil array includes a first set of loop coils arranged in a first column and a second set of butterfly coils arranged in a second, adjacent column, wherein in the unfolded configuration, the first set of loop coils and the second set of butterfly coils overlap by less than a threshold amount, and in the folded configuration, the first set of loop coils and second set of butterfly coils fully overlap.
 8. The method of claim 5, wherein generating the one or more images of the second region of interest based on the received second plurality of MR signals includes generating one or more images of the second region of interest using parallel imaging. 